Nipah virus and Hendra virus are emerging viruses that are responsible for previously unrecognized fatal diseases in animals and humans. These viruses are closely related members of a new genus, Henipavirus, in the Paramyxoviridae family, a diverse group of large, enveloped, negative-sense RNA viruses, that includes a variety of important human and animal pathogens. The recent emergence of these two viruses appears to have been the result of exposure of new hosts precipitated by certain environmental and behavioral changes. Hendra virus was identified first, from cases of severe respiratory disease that fatally affected both horses and man. Subsequent to that appearance, an outbreak of severe febrile encephalitis associated with human deaths occurred in Malaysia. Later studies identified a Hendra-like virus, now known as Nipah virus; as the etiologic agent of that episode. These viruses are unusual among the paramyxoviruses in their abilities to infect and cause disease with high fatality rates in a number of host species, including humans, and are zoonotic Biological Safety Level-4 agents. Presently, the cat appears to be the ideal small-animal model capable of reproducing the pathology seen in infected humans.
Nipah and Hendra virus are NIAID select, category C viruses and possess several features which make them highly adaptable for use as biowarfare agents. For example, both readily grow in cell culture or embryonated chicken eggs, produce high unconcentrated titers near 1×108 TCID50/ml, (14), are highly infectious and transmitted via the respiratory tract (22, 27), and can be amplified and spread in livestock serving as a source for transmission to humans. Recent evidence also indicates that nosocomial transmissibility of NiV from patients with encephalitis to healthcare workers is possible (45, 60).
Fusion of the membrane of enveloped viruses with the plasma membrane of a receptive host cell is a prerequisite for viral entry and infection and an essential step in the life cycle of all enveloped viruses. Research towards dissecting and understanding the mechanisms of this process is an important area of work. Not only does it afford insights into the complex interactions between viral pathogens and their host cells, but it can also shed light on the complex and essential biochemical process of protein-mediated membrane fusion, and also lead to the development of novel intervention and vaccine strategies. This has been demonstrated in the HIV research field, where the discovery of the long-sought coreceptors involved in entry and infection has opened a broad new era in the development of therapeutics to block the infection process at the level of entry (reviewed in (3, 18)).
Paramyxoviruses are negative-sense RNA enveloped viruses and encompass a variety of important human and animal pathogens, including measles virus (MeV), mumps virus, Sendai virus (SeV), Newcastle disease virus (NDV), rinderpest virus, canine distemper virus (CDV), human parainfluenza viruses (hPIV) 1-4, respiratory syncytial virus (RSV), and simian virus 5 (SV5) (reviewed in (36)). In contrast to retroviruses, paramyxoviruses contain two principal membrane-anchored glycoproteins, which appear as spikes projecting from the envelope membrane of the viral particle when viewed under the electron microscope. One glycoprotein is associated with virion attachment to the host cell, and, depending on the particular virus, has been designated as either the hemagglutinin-neuraminidase protein (HN), the hemagglutinin (H), or the G protein which has neither hemagglutinating nor neuraminidase activities (reviewed in (44)). The other glycoprotein is the fusion protein (F) which is directly involved in facilitating the fusion of the viral and host cell membranes (reviewed in (36)). Following virus attachment to a permissive host cell, fusion at neutral pH (or independently of the pH) between the virion and plasma membranes ensues, resulting in delivery of the nucleocapsid into the cytoplasm. In a related process, cells expressing these viral glycoproteins on their surfaces can fuse with receptor-bearing cells, resulting in the formation of multinucleated giant cells (syncytia) under physiological or cell culture conditions.
The Envelope Glycoproteins.
The HN envelope glycoprotein is responsible for attachment of the virion to its receptor, sialic acid, on the target cell as is the case for the hPIVs, NDV, SV5 and others. In contrast, the morbilliviruses, like MeV and CDV, have an attachment protein (H) possessing only hemagglutinating activity and do not bind to sialic acid. MeV was the first morbillivirus shown capable of utilizing a cell-surface protein as a receptor (19, 47), and was the demonstration of the predicted interaction between the MeV H glycoprotein and the MeV receptor CD46 using co-ip experiments and soluble CD46 (48). In addition, MeV field isolates as well as vaccine strains have been shown capable of utilizing signaling lymphocyte activation molecule (SLAM; CD150) (61). SLAM is also capable of serving as a receptor for several other morbilliviruses, including CDV (62).
A third class of paramyxovirus attachment glycoproteins, which are possessed by the Pneumovirinae such as RSV, are designated G, and have neither hemmagglutinating nor neuraminidase activities (reviewed in (44)). The attachment glycoproteins are type II membrane proteins where the molecule's amino (N)-terminus is oriented towards the cytoplasm and the protein's carboxy (C)-terminus is extracellular. The other major envelope glycoprotein is the fusion (F) glycoprotein, and the F of these viruses are more similar, where in all cases it is directly involved in mediating fusion between the virus and host cell at neutral pH.
The F glycoprotein of the paramyxoviruses is a type I integral membrane glycoprotein with the protein's N-terminus being extracellular. It shares several conserved features with other viral fusion proteins, including the envelope glycoprotein (Env) of retroviruses like gp120/gp41 of HIV-1, and hemagglutinin (HA) of influenza virus (reviewed in (26)). The biologically active F protein consists of two disulfide linked subunits, F1 and F2, that are generated by the proteolytic cleavage of a precursor polypeptide known as F0 (reviewed in (34, 55)). Likewise, HIV-1 Env and influenza HA are proteolytically activated by a host cell protease, leading to the generation of a membrane distal subunit analogous to F2 and a membrane-anchored subunit analogous to F1. In all cases, the membrane-anchored subunit contains a new N-terminus that is hydrophobic and highly conserved across virus families and is referred to as the fusion peptide (reviewed in (30)). All paramyxoviruses studied to date require both an attachment and F protein for efficient fusion, with the exception of SV5 which can mediate some fusion in the absence of HN (50). Evidence of a physical association between these glycoproteins has been observed with only limited success and only with NDV (57), hPIV (73), and recently with MeV (51), but these observations have often been with the aid of chemical cross-linking agents. It is hypothesized that following receptor engagement, the attachment protein must somehow signal and induce a conformational change in F leading to virion/cell fusion (35, 53). That conformational distinctions existed in the HN and F of a paramyxovirus depending on whether they were expressed alone or in combination has been noted for quite sometime (13).
The Paramyxovirus F envelope glycoproteins, like those of retroviruses, are considered class I membrane fusion-type proteins. An important feature of the fusion glycoproteins of these viruses is the presence of 2 α-helical domains referred to as heptad repeats that are involved in the formation of a trimer-of-hairpins structure during or immediately following fusion (29, 56). These domains are also referred to as either the amino (N)-terminal and the carboxyl (C)-terminal heptad repeats (or HR1 and HR2), and peptides corresponding to either of these domains can inhibit the activity of the viral fusion glycoprotein when present during the fusion process, first noted with sequences derived from the gp41 subunit of HIV-1 envelope glycoprotein (32, 67). Indeed, HIV-1 fusion-inhibiting peptides have met with clinical success and are likely to be the first approved fusion inhibitor therapeutics. Peptide sequences from either the N or C heptads of the F of SV5, MeV, RSV, hPIV, NDV, and SeV have also been shown to be potent inhibitors of fusion (33, 37, 52, 68, 74, 75). It is generally accepted that significant conformational change would occur during activation of paramyxovirus F fusogenic activity. Differential antibody binding reactivities of precursor and proteolytically processed forms of SV5 F (20) and in conjunction with the structure of the post fusion 6-helix bundle of SV5 F (2), strongly support the conformational change model, not only from the pre-fusion to post-fusion structural change, but also from the F0 precursor to the F2-F1 mature protein. That the post-fusion structure of a paramyxovirus F core is likely conserved across other paramyxoviruses has been further supported by the F core structures of RSV (79) and MeV (80). However, recent structural studies on the F glycoprotein of NDV have yielded some different and interesting findings. The oligomeric trimer structure of NDV F (in perhaps the pre-fusion or meta-stable state) has offered some alternative information which distinguishes it from the classic influenza HA structure, this is principally reflected in the completely opposite orientation of the central coiled coils formed by the HR1 (also termed HRA) segments of the trimer (9, 10). To date this is the only structural information on the pre-fusion (or meta-stable) form of a paramyxovirus F (in fact the only other meta-stable, class I, structure other than influenza HA), and perhaps represents a possible third-class of viral fusion proteins.
A precise understanding of how the fusion and attachment glycoproteins function in concert in mediating fusion has yet to be elucidated, but there are two central models proposed for the role of the attachment glycoprotein in the paramyxovirus-mediated membrane fusion process, which were recently detailed by Morrison and colleagues (41), in the context of the HN glycoprotein of NDV. In the first model, the fusion and attachment glycoproteins are not physically associated in the membrane, but following receptor engagement there is an alteration in the attachment glycoprotein which facilitates its association with F and in so doing imparts or facilitates F conformational change leading to membrane fusion. In the second model, the F and attachment glycoprotein are pre-associated and receptor engagement induces conformational alteration in the attachment glycoprotein, and this process alters or releases an interaction with F that allows F to proceed towards its fusion active state—formation of the 6-helix bundle just prior or concomitant with membrane merger. Findings on NDV demonstrate the variable accessibility of the HR1 domain during the process, where HR1 of F are accessible to specific fusion-inhibiting antibody when F is presented in the context of HN, however expression of F alone results in a non-fusogenic version of F with distinctly altered conformation having an HR1 domain which is no longer accessible to antibody (41). The second model is that the attachment glycoprotein is holding F in its non-fusogenic conformation and upon receptor engagement and conformational change in the attachment glycoprotein F is released to undergo conformational changes leading to 6-helix bundle formation and facilitation of membrane fusion. This is supported by observations that paramyxovirus F expressed alone neither mediates fusion (with the exception of SV5 under certain conditions) and has variably antibody accessibility of certain domains such as the NDV F HR1 domain (41). This is perhaps because F alone has transitioned to a fusion triggered or intermediate conformation at an inappropriate moment, which would be consistent with observations of fusion defective or triggered HIV-1 gp41 also referred to as dead-spikes (17). Preliminary findings with HeV and NiV support this second model. Finally, that the attachment glycoprotein of a paramyxovirus undergoes specific conformation alteration when bound to receptor has been recently revealed at the molecular level from studies on the HN glycoprotein of NDV (58, 59). These studies have revealed clear differences in the structure of HN when the receptor-bound glycoprotein is compared to the non-receptor-bound HN structure. In addition, all known viral envelope glycoproteins are homo- or heterooligomers in their mature and functional forms (reviewed in (16)). Multimeric proteins, like these, generally interact over large areas, making structural differences between monomeric subunits and the mature oligomer likely (31). This feature can also translate into differences in antigenic structure and has been shown for a number of proteins, most notably the trimeric influenza HA glycoprotein (69) and HIV-1 gp120/gp41 (7). Indeed, a trimer-specific, potent neutralizing determinant has been mapped to the interface between adjoining subunits of HA, and oligomer-specific anti-HIV-1 Env antibodies have been identified and mapped to conformation-dependent epitopes in gp41 (7). Thus far, all paramyxoviruses, retroviruses, and influenza virus fusion glycoproteins appear to be homotrimers (8, 9, 21, 54, 71), and several HN attachment proteins have been shown to be tetrameric, comprised of a dimer of homodimers. For example, the NDV HN can form dimers and tetramers on the viral surface (40, 43), and recently the crystal structure of the globular head region of the HN dimer from NDV has been solved (15). Finally, and of importance in understanding certain aspects of the immune response to these viruses and the development of vaccines, it is these major envelope glycoproteins of these viruses to which virtually all virus-neutralizing antibodies are directed against.
Emerging Pathogenic Paramyxoviruses.
In 1994, a new paramyxovirus, was isolated from fatal cases of respiratory disease in horses and humans, and was shown to be distantly related to MeV and other members of the morbillivirus genus; it was provisionally termed equine morbillivirus (EMV) but has since been re-named Hendra virus (HeV) (46). The first outbreak of severe respiratory disease in the Brisbane suburb of Hendra Australia resulted in the death of 13 horses and their trainer, and the non-fatal infection of a stable hand and a further 7 horses. At approximately the same time, in an unrelated incident almost 100 km north of Hendra, a 35-year-old man experienced a brief aseptic meningitic illness after caring for and assisting at the necropsies of two horses subsequently shown to have died as a result of HeV infection. Thirteen months later this individual suffered a recurrence of severe encephalitis characterized by uncontrolled focal and generalized epileptic-activity. A variety of studies that were performed in the evaluation of this fatality, including serology, PCR, electron microscopy (EM) and immunohistochemistry, strongly suggested that HeV was indeed the cause of this patient's encephalitis, and the virus was acquired from the HeV-infected horses 13 months earlier (49). In all, fifteen horses and two people died in the two episodes. At the time the source of the emerging virus was undetermined, but more recently it has been found that approximately 50% of certain Australian fruit bat species, commonly known as flying foxes, have antibodies to HeV and Hendra-like viruses have been isolated from bat uterine fluids. It appears that these animals are the natural host for the virus (22, 24, 25, 76). Recently, the nucleic acid sequence of HeV genes has been analyzed and compared with those of other paramyxoviruses (64, 77, 78). These studies have confirmed that HeV is a member of the Paramyxoviridae, subfamily Paramyxovirinae.
Subsequent to these events, an outbreak of severe encephalitis in people with close contact exposure to pigs in Malaysia and Singapore occurred in 1998 (1). The outbreak was first noted in September 1998 and by mid-June 1999, more than 265 cases of encephalitis, including 105 deaths, had been reported in Malaysia, and 11 cases of disease with one death reported in Singapore. This outbreak had a tremendous negative economic impact, which continues to date. Although successful, measures taken in the early days of the outbreak resulted in the slaughter of approximately 1.3 million pigs and the virtual closure of the pig farming industry in peninsular Malaysia. EM, serologic, and genetic studies have since indicated that this virus is also a paramyxovirus, and was closely related to HeV. This virus was named Nipah virus (NiV) after the small town in Malaysia from which the first isolate was obtained from the cerebrospinal fluid of a fatal human case (11, 12, 23, 38, 39).
Most human patients present with acute encephalitis, which in the Malaysia outbreak of 1998-1999 ultimately resulted in a mortality rate of approximately 40%, but infection can also present as a nonencephalitic or asymptomatic episode with seroconversion. Interestingly, infection with NiV can also take a more chronic course with more serious neurological disease occurring late (greater than 10 weeks) following a nonencephalitic or asymptomatic infection. On the other hand, the recurrence of neurological manifestations (relapsed encephalitis) has also been noted in patients who had previously recovered from acute encephalitis. Cases of relapsed-encephalitis presented from several months to nearly two years after the initial infection (72) Taken together, there was nearly a 10% incidence rate of late encephalitic manifestations with a mortality rate of 18%. Thus, with both NiV and HeV a prolonged period of infection is possible before serious neurological disease occurs. The underlying mechanisms which allow these viruses, especially NiV, to escape immunological clearance for such an extended period are completely unknown.
In the case of NiV, the late presentation of neurological disease and IgG subclass response showed similarities to subacute sclerosing panencephalitis (SSPE), a rare late manifestation of MeV infection (72). It was molecular characterization of HeV and NiV which distinguished them as distinctly new paramyxoviruses. The families Paramyxoviridae, Filoviridae, Rhabdoviridae, and Bornaviridae are all negative-sense RNA enveloped viruses sharing similar genome organization, replication strategies, and polymerase domain structure (63). These families are grouped in the order Mononegavirales, the first taxon above family level virus taxonomy. The genome size in the Mononegavirales is wide ranging, ˜8.9-19.1 kb. The genomes of paramyxoviruses, as a group, are generally considered tightly spread, having sizes in the range of 15.1-15.9 kb, except HeV and NiV whose genomes of 18.2 kb, far closer in size to the Filoviridae. Much of this added length is untranslated regions at the 3′ end in the six transcription units, again quite similar to Marburg and Ebloa Filoviruses (63). Also, the P protein is larger by 100 residues (longest known), and a small basic protein (SB) in HeV of unknown function. Taken together, the molecular features of both HeV and NiV make them unusual paramyxoviruses, as does their ability to cause potentially fatal disease in a number of species, including humans.
Pathogenesis.
The development or characterization of animal models to study these newly identified viral zoonoses is important for understanding their pathogenic features and in the development of therapeutics. Of the two fatal cases of HeV infection in humans, the first was the result of severe respiratory disease following several days of ventilated life-support. The patient's lungs had gross lesions of congestion, hemorrhage and oedema associated with histological chronic alveolitis with syncytia. The second fatal case was one of leptomeningitis with lymphocytes and plasma cells and foci of necrosis in various parts of the brain parenchyma, after initial infection more than 1 year previously (reviewed in (27)). Multinucleate endothelial cells were also seen in the viscera as well as in the brain. In contrast, there were many more human cases of infection with NiV. More than 30 individuals resulting from the large NiV outbreak in Malaysia and Singapore were autopsied, and the immuno- and histological features included systemic endothelial infection accompanied by vasculitis, thrombosis, ischaemia and necrosis (reviewed in (27)). These changes were especially noted in the central nervous system (CNS). Immunohistochemical analysis have also shown widespread presence of NiV antigens in neurons and other parenchymal cells in necrotic foci seen in the CNS as well as in endothelial cells and media of affected vessels (27). In infected humans, evidence of vasculitis and endothelial infection was also seen in most organs examined. Disseminated endothelial cell infection, vasculitis, and CNS parenchymal cell infection play an essential role in the fatal outcome of NiV infection in humans (reviewed in (27)). The principal zoonotic episodes in nature involved the horse in the HeV cases and the pig in the case of NiV. Both these viruses have a notable broad host cell tropism in in vitro studies (4, 5). These observations correlated to what has been observed in natural and experimental infection.
Experimental infections of the horse and pig have been carried out with HeV and NiV respectively and one naturally NiV-infected horse has been examined. The pathology caused by either virus in horses appears to be more severe than that caused by NiV in pigs. In addition to pigs, HeV infection of cats has also been performed and in this case disease resembles that seen in horses, characterized by generalized vascular disease with the most severe effects seen in the lung (28). Guinea pigs have also been experimentally infected with HeV (28) and the pathology seen differed significantly in several respects in comparison to the human cases as well as natural and experimentally infected horses. In guinea pigs HeV caused generalized vascular disease but, unlike horses and cats, there was little or no pulmonary oedema. Histologically, vascular disease was prominent in arteries and veins, and in many organs such as the lung, kidney, spleen, lymph nodes, gastrointestinal tract and skeletal and intercostal muscles. NiV infection of the guinea pig has not yet been well described.
In regards to other small laboratory animal models, NiV and HeV do not cause disease in mice even after subcutaneous administration, however, and not surprisingly, they will kill mice if administered intracranially. Further, there is also no serological evidence for NiV in rodents in Malaysia, and several hundred sera were tested during the outbreak. Evidence of natural NiV infections were also noted in dogs and cats.
In experimental NiV infection of the cat, gross lesions in animals with severe clinical signs strongly resembled those of cats infected with HeV. These consisted of hydrothorax, oedema in the lungs and pulmonary lymph nodes, froth in the bronchi, and dense purple-red consolidation in the lung. There were also similar features in the histological appearance, diffuse perivascular, peribronchial and alveolar hemorrhage and oedema, vasculitis affecting arteries and arterioles, alveolitis, syncytium formation within endothelial cells and alveolar epithelial cells (reviewed in (27)). Taken together, the evidence to date indicates that the cat represents an animal model whereby the pathology seen most closely resembles the lethal human disease course. In addition, infection of cats with either NiV or HeV is uniformly fatal. NiV and HeV appear to cause similar diseases but with some notable variations, and although the basic pathologic processes have been well described, less is known about the factors which clearly influence disease course depending on the species infected. This is a special concern in human infections, where there is a remarkable ability of these viruses to persist in the host (up to 2 yrs) before causing a recurrence of severe and often fatal disease. Cats succumb within 6-8 days to subcutaneous infection with 5,000, and subcutaneous or oral administration of 50,000, TCID50 of a low passage, purified HeV (65, 66, 70). Experimental infection of cats with NiV has confirmed the susceptibility of this species to oronasal infection with 50,000 TCID50 NiV (42). In summary, the clinical and pathological syndrome induced by NiV in cats was comparable with that associated with HeV infection in this species, except that in fatal infection with NiV there was extensive inflammation of the respiratory epithelium, associated with the presence of viral antigen.
In summary, recurrent outbreaks of NiV resulting in significant numbers of human fatalities have recently been confirmed (Fatal fruit bat virus sparks epidemics in southern Asia. Nature 429, 7, 6 May 2004). HeV is also know to cause fatalities in human and animals and is genetically and immunologically closely related to NiV. There are presently no vaccines or therapeutics for prevention of infection or disease caused by Nipah virus or Hendra virus. Both Nipah virus and Hendra virus are United States, National Institute of Allergy and Infectious Disease, category C priority agents of biodefense concern. Further, as these viruses are zoonotic Biological Safety Level-4 agents (BSL-4), production of vaccines and/or diagnostics, with safety is very costly and difficult. Thus, there is a need for a Nipah virus or Hendra virus vaccines and diagnostics that allow for high throughput production of vaccines and/or diagnostics. All references cited herein, including patent applications and publications, are incorporated by reference in their entirety.